In our last list of punctuation challenges, we looked at troublesome issues like ellipses, hyphens, serial commas, and quotation marks. But obviously those aren't the only punctuation marks that can be confusing for writers. Below are four more issues that can leave students and professionals alike scratching their heads.

1. One Space or Two?

The one or two space issue isn't as complicated as some of the other punctuation questions we're going to be looking at, but it's one of the most common. It used to be that, when people used typewriters, everyone put two spaces between sentences. Because typewriter font is monospaced-meaning all the letters take up the same amount of space-it was necessary to put two spaces between the period that ended a sentence and the start of the next one. This extra white space provided a visual guide to readers so they could easily see the break between sentences.

With the advent of computer word processing programs came proportional fonts, which allocated different amounts of space for different letters (for example, the letter l takes up less space than the letter h). With these fonts, it is no longer necessary to include the extra space between sentences. For comparison, here is the same sentence in Courier, a monospace font, and Arial, a proportional font (also note how much longer the monospaced sentence is):

Monospaced

Don't forget to feed the dog. He looks hungry.

Proportional

Don't forget to feed the dog. He looks hungry.

What does all this mean for your writing? While some teachers might still fight on the two spaces side of the debate, most style manuals, including Chicago, APA, and MLA, require only one space between sentences. So, unless you're told otherwise, it's probably best to use only one.

2. Periods and Parenthesis

Parenthesis have a number of uses in formal writing. They're often used to cite sources or figures, and they're also used to as a way to include information that's interesting or relevant but not vital to the main ideas of the paper. When using parenthesis for this second purpose, it's important to know how to punctuate your sentences.

When it comes to parenthesis, the rules of punctuation are actually pretty straightforward (particularly when compared to some of the other issues on this list). If the sentence contained within the parenthesis is not a complete sentence, then the ending punctuation goes outside the closing parenthesis. Since the idea inside the parenthesis is not a full sentence of its own (like the first sentence of this paragraph), it doesn't get its own punctuation.

However, if the sentence inside the parenthesis is a clause that can stand on its own, then it does get its own punctuation. In this case, the preceding sentence will have its own ending punctuation, and the period (or question mark/exclamation point) that ends the sentence inside the parenthesis will stay inside the closing parenthesis as well. (Such a sentence would look like this.)

3. Possessive Plurals

Apostrophes are a tricky punctuation mark for a lot of writers because the rules for their use are pretty elaborate. To talk about apostrophes, first we have to cover what exactly they're used for. In English, apostrophes serve two purposes. The first is to indicate that letters are have been omitted from a word; this is most commonly seen in contractions, for example "she is walking" becomes "she's walking." While contractions can create their own problems, here we're going to concerns ourselves with the other, unrelated reason to use apostrophes, which is to show possession.

Creating the possessive of a basic, singular noun is pretty straightforward: you just add an 's to the end. "The books the belong to the girl" becomes "the girl's books," and "the desk of a student" becomes "a student's desk."

That was pretty simple, right? But problems start to crop up quickly when we start making other word's possessive. For instance, what do you do with a word that already ends with the letter s, like boss or octopus? If this is a question you've struggled with before, you shouldn't feel too bad, because it turns out there isn't one correct answer. Some style guides require only an apostrophe, so "the chair that belongs to my boss" becomes "the boss' chair," but other require an 's, so "the legs of the octopus" becomes "the octopus's legs." Which one you use will depend on the style manual you're following, so always check if you're not sure.

When it comes to plurals the rules are a bit clearer. For words like tables and cars, you only need to add the apostrophe; no extra s is needed. So, "the legs of the tables" becomes "the tables' legs" and "the wheels on the cars" become "the cars' wheels." However, words with unconventional spellings for plurals, like mice and children, do require the 's: mice's cheese, children's toys.

4. Vertical Lists

Everybody loves a list. Certainly in the age of the internet, we all spend a lot of time looking at Top Tens and Five Favorites, and lists can also be useful in academic writing. Punctuating lists, though, creates a unique set of challenges. Many style guides don't address punctuating lists directly, but a few, mainly the Chicago Manual of Style, have a few pointers.

The first issue when it comes to punctuating lists is the introductory sentence. Most lists are going to be introduced with a phrase like "There are three reasons students choose to include lists in their papers" or "Why it's important to recycle plastic bottles." When using these types of headers, you can choose to either include a colon or no punctuation. Generally, style guides recommend that you use a colon after a complete sentence, but don't punctuate other types of introductions. So, the lists from the example above would look like this:

There are three reasons students choose to include lists in their papers:

Lists are a good way to condense lots of information.

Lists draw the reader's attention.

Lists help group together important ideas.

Why it's important to recycle plastic bottles

It takes less resources to produce new bottles from recycled materials.

Recycling plastic saves landfill space.

Reusing plastics lowers the demand for petroleum.

You also might have noticed that in all the examples above, each item ends with a period. That's because each point is a complete sentence. When the items in a list aren't complete sentences, it's not necessary to add punctuation at the end of each one.

Classes students take freshmen year include:

Algebra

English

History

Biology

Lastly, keep in mind that it's not necessary to use commas and conjunctions when writing a list. The items in your list aren't technically part of the same sentence, so it's not necessary to write them like they are.

Incorrect

Students often have problems with:

Spacing after sentences,

Possessive plurals,

Vertical Lists, and

Periods and Parenthesis.

Correct

Students often have problems with:

Spacing after sentences

Periods and Parenthesis

Vertical Lists

Possessive plurals, and

While grammar might seems like it's set in stone, in fact many of the rules are pretty controversial. Most of the guidelines discussed here are particular to certain style guides, so it's always a good idea to check your MLA Handbook, Chicago Manual of Style, or whichever guide you're using in order to make sure that your use of punctuation is correct.

By Kevin Demlon.Kevin is an experienced freelance writer. He does not waste time and does his best to provide high-quality writing manuals and useful samples.